We report the extracellular production of superoxide in cultures of the marine diatoms Thalassiosira weissflogii and Thalassiosira pseudonana. In EDTA-buffered media with ∼45 pmol L-1 iron (Fe)(III)′, over half the Fe(III) reduction is mediated by extracellular O2 - production. Surprisingly, even though we saw a consistent inhibition of Fe reduction by the addition of superoxide dismutase (SOD) and enhancement of Fe reduction due to superoxide production using the xanthine-xanthine oxidase system (X-XO), we observed no effect of SOD or X-XO on Fe uptake in these cultures. We also observed no effect of SOD on the uptake of Fe from either ferrihydrite, an Fe-labeled porphyrin, or as regenerated by metazoan grazers. Our data reveal that Fe(II)′ is formed by O 2- via the reduction of Fe(III)′ rather than by the reduction and dissociation of FeEDTA complexes in EDTA-buffered media. This bulk-phase reduction of Fe(III)′ by O2- is a "futile" reductive step, as the Fe(II)′ formed is rapidly reoxidized to Fe(III)′ in pH 8 oxygenated seawater.